This invention relates to resonators and, more particularly, to plate wave resonators that may be part of a micro electro mechanical system.
Various types of resonators including surface acoustic wave (SAW) crystals, quartz oscillators, flexure plate wave devices, are known. However, each known type of resonator has its drawbacks.
SAW devices are by nature relatively large, on the order of a centimeter, due to their long wavelength and the need for many reflecting fingers at each end to trap surface acoustic energy. The mass involved in the resonant motion of a SAW device is relatively large, on the order of one acoustic wavelength in depth times the product of area of the device and the density. Plate wave devices have a much smaller mass involved in the resonant motion since the thickness of the plate wave device is typically much smaller than a wavelength of the acoustic wave. The large mass involved in SAW oscillation makes them less sensitive to small mass perturbations than plate wave oscillators. Due to their high frequency, SAW devices also consume more power than lower frequency resonators.
Quartz resonators also have the disadvantage that the mass involved in oscillation is quite large, resulting in low sensitivity to small mass perturbations. This holds for shear mode, thickness mode, and tuning fork quartz oscillators.
Known plate wave resonators are capable of oscillating in many closely spaced modes. This capability makes it difficult for circuits including the resonators to stabilize the resonators on a single mode and results in xe2x80x9cmode hoppingxe2x80x9d or frequency instability. Frequency instability is a recurring problem for known plate wave resonators. For example, mode hopping represents a noise source which limits the reliability of the oscillation mode of the resonator as a reference. Also, when resonators are used as sensors, frequency instability reduces their detection sensitivity. Apodization of the electrodes associated with the resonators has been used to ameliorate frequency instability. Apodization, however, leaves several modes of equal amplitude in which a resonator can oscillate and is therefore at best only a partial solution.
Accordingly, the need for a small, low power, single mode micro-machined resonator remains.
In one aspect, the invention features a resonator with mechanical node reinforcement comprising a substrate, an intermediate portion, and a resonant portion. The intermediate portion is adjacent to the substrate. The resonant portion is adjacent to the intermediate portion and is defined by a periphery. The resonant portion is also adapted for a first oscillation mode with a nodal point located within the periphery; the resonant portion contacts the intermediate portion only at points located substantially at the periphery and substantially at the nodal point.
In a similar aspect, the invention features a resonator with mechanical node reinforcement comprising a substrate and a resonant portion. The resonant portion is adjacent to the intermediate portion and is defined by a periphery. The resonant portion is also adapted for a first oscillation mode with a nodal point located within the periphery. According to one preferred embodiment, the resonant portion contacts the substrate at points located substantially at the periphery and substantially at the nodal point.
Embodiments of the two foregoing aspects of the invention may include the following features: a contact point located substantially at the nodal point that interferes with an unwanted oscillation mode; a plurality of nodal points within the periphery substantially at which the resonant portion contacts the underlying structure; and a resonant portion affixed to the underlying structure substantially at the nodal point and, in some embodiments, at the periphery.
In some embodiments, the resonant portion and/or the intermediate portion comprises layers of materials selected for a particular means of exciting the oscillation mode. In one embodiment, for example, the intermediate portion comprises a conductive layer and the resonant portion comprises a structural layer adjacent to the intermediate portion and an electrode layer adjacent to the structural layer. In another embodiment, the resonant portion comprises a structural layer adjacent to the intermediate portion, a conductive layer adjacent to the structural layer, a piezoelectric layer adjacent to the conductive layer, and an electrode layer adjacent to the piezoelectric layer. In another embodiment, the piezoelectric layer of the foregoing embodiment is replaced with a ferroelectric layer.
In some embodiments, the resonator is adapted for a particular application. In one embodiment, for example, the resonator further comprises a drive transducer coupled to the resonant portion, and adapted for converting a first electrical signal into a wave in the resonant portion, and a sense transducer coupled to the resonant portion, and adapted for converting the wave in the resonant portion into a second electrical signal. In another embodiment, the resonator further comprises an amplitude control device coupled to the sense transducer, and adapted for calculating a variance by comparing an amplitude represented by the second electrical signal to an amplitude set point, and a phase shift device coupled to the drive transducer, and adapted for transmitting a third electrical signal to the drive transducer based on the variance. In another embodiment, the resonator further comprises a film on the resonant portion that has at least one property that changes upon exposure to an agent and affects the first oscillation mode thereby enabling the resonator to operate as a sensor.
In another aspect, the invention features a method of manufacturing a resonator with mechanical node reinforcement comprising applying an intermediate portion to a substrate, applying a sacrificial layer to the intermediate portion, applying a resonant portion to the sacrificial layer and the intermediate portion. The sacrificial layer is defined by a periphery, and is particularly adapted for an oscillation mode with a nodal point located within the periphery. The sacrificial layer also has a void located substantially at the nodal point. The resonant portion extends beyond the periphery of the sacrificial layer and contacts the intermediate portion at the void. This aspect also features removing the sacrificial layer such that a gap remains between the intermediate portion and the resonant portion.
In a similar aspect, the invention features a method of manufacturing a resonator with mechanical node reinforcement comprising applying a sacrificial layer to a substrate, then applying a resonant layer to the sacrificial layer and the substrate. The sacrificial layer is defined by a periphery, and particularly adapted for an oscillation mode with a nodal point located within the periphery; the sacrificial layer has a void located substantially at the nodal point. The resonant layer extends beyond the periphery of the sacrificial layer and extends through the sacrificial layer to contact the substrate at the void. This aspect also features removing the sacrificial layer such that a gap remains between the substrate and the resonant layer.
Embodiments of the two foregoing aspects of the invention may include the following features: creating the void in the sacrificial layer by etching the sacrificial layer; sealing the gap in a vacuum; creating a sacrificial layer with a plurality of voids located substantially at a plurality of nodal points associated with a desired oscillation mode and located within the periphery of the sacrificial layer; and affixing the resonant portion to the sacrificial layer and to the intermediate portion or, if no intermediate portion is applied, to the substrate.
In some embodiments the method of manufacturing the resonator is adapted for a particular means of exciting the oscillation mode. In one embodiment, for example, the method further comprises applying a conductive layer to the substrate, applying a structural layer to the sacrificial layer and the intermediate portion, and applying an electrode layer to the structural layer. In another embodiment, the method further comprises applying a structural layer to the sacrificial layer and to the intermediate portion or, if no intermediate portion is applied, to the substrate, applying a conductive layer to the structural layer, applying a piezoelectric layer to the conductive layer, and applying an electrode layer to the piezoelectric layer. In another embodiment, the application of the piezoelectric layer in the foregoing embodiment is replaced by the application of a ferroelectric layer.
In some embodiments, the method of manufacturing is adapted for a particular application. In one embodiment, for example, the method further comprises applying a drive transducer to the resonant portion, the drive transducer converting a first electrical signal into a wave in the resonant portion, and applying a sense transducer to the resonant portion, the sense transducer converting the wave in the resonant portion into a second electrical signal. In another embodiment, the method further comprises putting an amplitude control device in electrical signal communication with the sense transducer, the amplitude control device calculating a variance by comparing an amplitude represented by the second electrical signal to an amplitude set point, and putting a phase shift device in electrical signal communication with the drive transducer, the phase shift device transmitting a third electrical signal to the drive transducer based on the variance. In another embodiment, the method further comprises applying to the resonant portion a film with at least one property that changes upon exposure to an agent and that affects the oscillation mode of the resonant layer.
The foregoing and other objects, aspects, features, and advantages of the invention will become more apparent from the following description and from the claims.